Bitumen is used as a binder in road asphalt mixtures, and has developed continually to meet ever-increasing performance demands from road building constructors. In general bitumen performs well in road asphalt, but increasingly heavy traffic loads have led to the premature wear of many roads through rutting and cracking of the surface. Cracking is a serious defect in a road asphalt because it allows water to reach lower layers of the road surface where it causes rapid deterioration, and accelerates the need for premature repairs. Increasing the bitumen content of asphalt or using a softer grade of bitumen improves the crack resistance of asphalt at low temperatures but increases the risk of excessive rutting at higher temperatures because the mixture is effectively softer. Conversely, resistance of rutting can be improved by reducing the amount of bitumen in the asphalt mixture or by using a harder grade of bitumen, at the expense of crack resistance because the mixture becomes less flexible.
In view of the above it will be clear that it would be advantageous to develop a hard bitumen composition meeting today's crack resistance requirement, i.e. a bitumen composition having both a good low temperature performance and a good high temperature rutting resistance.
It is known that the low temperature properties of bitumen can be improved by blending them with a polymer. However, when this modification is applied to hard bitumen, an incompatibility between the bitumen and the polymer is generally observed, resulting in hardly or not improved low temperature properties and a relatively poor ageing behaviour.
It is further known to prepare industrial and roofing-grade bitumen by subjecting a bitumen/polymer mixture to a conventional blowing process. These bitumen compositions, however, appear to be not suitable for road applications, due to their high softening point and relatively high penetration.